The long-term reliability and safety of implantable cardiac leads is a significant issue. Anomalies of conductors in implantable medical devices constitute a major cause of morbidity. Representative examples of such medical devices include, but are not limited to, pacemakers, vagal nerve stimulators, pain stimulators, neurostimulators, and implantable cardioverter defibrillators (ICDs). “Multilumen” ICD defibrillation leads include one or more high-voltage conductors and one or more pace-sense conductors. The leads can be implanted as subcutaneously or intravascularly. The lead body consists of a flexible, insulating cylinder having three to six longitudinal lumens configured in parallel. Conductors are disposed in the lumens providing an electrical pathway between the proximal terminals to small pace-sense electrodes and larger shock coil electrodes.
Early diagnosis of ICD lead conductor anomalies is important to reduce morbidity and/or mortality. Anomalies may occur in either the pace-sense or shock components. The most common presentation of pace-sense failures is oversensing of rapid, nonphysiological signals. Prior to the implementation of diagnostics that incorporated oversensing, pace-sense fractures presented most commonly as inappropriate shocks, despite daily, automated measurements of pacing impedance. Functional failure of a pace-sense conductor or oversensing also results in inhibition of bradycardia pacing, cardiac resynchronization, or antitachycardia pacing. Lead insulation failure or fractures may also present with loss of capture. High-voltage conductor failures present with potentially fatal failed defibrillation or cardioversion. Thus, early diagnosis of ICD lead conductor anomalies is a critically important step in reducing morbidity and/or mortality from the failure of pacing, inappropriate ICD shocks, and/or ineffective treatment of ventricular tachycardia or fibrillation (ventricular fibrillation). The early diagnosis of conductor anomalies for implantable cardiac leads is a critically important step in reducing these issues and making ICDs safer.
Thus, one major goal is high sensitivity of diagnosis: identification of lead conductor failures at the subclinical stage, before they present as a clinical problem. A second major goal is high specificity: a false positive provisional clinical diagnosis of lead insulation failure may trigger patient anxiety and lead to potentially avoidable diagnostic testing. A false positive clinical diagnosis of insulation failure results in unnecessary lead replacement, with corresponding expense and surgical risk.
Currently, the primary method, of which various methods are well-known in the art, for monitoring pacemaker and ICD lead integrity is periodic measurement of electrical resistance, commonly referred to as “impedance monitoring.” The current art impedance monitoring methods use single pulses and provide a value of impedance close to the direct-current resistance.
For pace-sense conductors, the circuit being measured comprises the connection between the implantable pulse generator header and the lead, the conductors to the tip and ring electrodes, and the electrode-myocardial interface. Most of the resistance is at the electrode-tissue interface of the high-resistance tip electrode, and variations of up to 10% in this value are common. Each individual pace-sense conductor (for example, the conductor to the tip electrode or the ring electrode) contributes less than 10% to the measured resistance. Thus, even if the pace-sense resistance in a single conductor doubled or tripled, the overall measured resistance remains within the expected range.
In fact, with presently-used methods, abnormal increases in impedance occur before diagnostic oversensing—and usually shocks—in only 28% of confirmed Fidelis fractures. For example, the normal resistance of the ring connecting cable (in a Riata® lead) is only 13Ω. This connection has two cables in parallel, with each cable having a normal resistance of 26Ω. In the case of one of the two cables being completely severed, the impedance thus only increase to 26Ω while the impedance seen by the conventional lead impedance “monitoring”—which includes the impedance of the electrode-electrolyte impedance and the return through another electrode—is around 500Ω to 2 kΩ and is thus undetectable. The normally sensed impedance will typically vary by hundreds of ohms during the course of the implant and thus a change of 13Ω is nominal and is easily missed.
Therefore, measurements indicate that resistance does not exceed the expected range until the conductor has lost most of its structural integrity and resistance remains within the expected range even when only a fraction of the conductor is intact.
Further, resistance measurements have limited specificity. A single, out-of-range value may be an artifact, and marked, persistent increases can occur at the electrode-myocardial interface in normally functioning leads, thus resulting in over diagnosis of lead failures.
Hafelinger et al. (U.S. Pat. No. 5,003,975) and Cinbis et al. (U.S. Pat. No. 5,897,577) summarize some of these methods, which include measurements made directly using either a single pacing pulse or a single independent pulse used only for measuring resistance. McVenes et al. (U.S. Pat. No. 5,741,311) describes use of a longer burst of alternating current at a single frequency. The purpose of these longer (about 100 ms) pulses is to drive the system to a steady-state condition that is not achieved by single, short (less than 1 ms) pacing pulse. Schuelke et al. (U.S. Pat. No. 5,755,742) describes a method for measuring resistance of defibrillation electrodes by applying a test voltage applied to a different excitation current pathway. Kroll et al. (U.S. Pat. No. 5,944,746) describes an automated method for periodic measurement of the resistance of the high-voltage (defibrillating) coil in ICD electrodes. Gunderson et al. (U.S. Pat. No. 7,047,083) describes a method and system for automated periodic measurements of resistance in conductors attached to an ICD or pacemaker. However, these types of “impedance monitoring,” which return values close to direct current resistance, identify lead anomalies before inappropriate shocks in only about a third of ICD patients who have conductor fractures.
A newer method for monitoring ICD lead integrity is based on the response of ICD pulse generators to electrical “noise” signals associated with lead conductor fractures. These nonphysiological signals have specific characteristics that differentiate them from true cardiac signals such as high variability and, at times, nonphysiologically-rapid rates. If these signals are of sufficient amplitude and exceed the ICD's dynamically-changing sensing threshold, the ICD oversenses them. Repetitive oversensing of nonphysiologically-short intervals may indicate lead conductor fracture even if lead resistance is normal. Gunderson et al. (U.S. Pat. No. 7,289,851) described a Lead-Integrity Alert that incorporates both ICD-based measures of oversensing based on the nonphysiologically-rapid rate of sensed signals and periodic measurements of resistance. This method, combined with automatic ICD reprogramming, improves warning time before inappropriate shocks caused by lead-related oversensing. Nevertheless, approximately 25% of patients receive less than 3 days of warning, and some receive almost no warning.
Additionally, the Gunderson Lead Integrity Alert method detects only some lead-noise signals. It cannot detect a lead anomaly unless it generates signals that are both fast enough and of sufficient amplitude to be classified as nonphysiological oversensing. Thus, it will not detect a lead anomaly if it does not generate “noise signals” or if it generates only low-amplitude noise signals, or signals that do not occur at a fast enough rate.
Gunderson et al. (U.S. Pat. No. 7,369,893) describes a method for withholding delivery of ICD shocks if ventricular fibrillation is detected from analysis of the pace-sense lead, but is not confirmed by analysis of the high-voltage lead. Although not yet evaluated in patients, this method is expected to further reduce unnecessary shocks. However, it requires sufficient oversensing to result in inappropriate detection of ventricular fibrillation clinically. Thus, it does not provide early diagnosis of conductor anomalies. Withholding shocks for ventricular fibrillation detected on the near-field electrogram has an inherent risk of withholding life-saving therapy, however small, and is, thus, not the preferred approach to diagnosis conductor fracture. Like the Lead-Integrity Alert method, it is not applicable to intraoperative diagnosis or to pacemakers and neurostimulators.
Comparable limitations apply to measuring impedance of the high-voltage lead components. The circuit measured comprises the connection between the implantable pulse generator header and lead, high-voltage conductor cables, shock electrodes, blood in the right atrium and ventricle, and—providing the CAN or ICD housing is included—heart, lung, and chest wall. High-voltage cables and shock electrodes have low impedance (˜1Ω), and the overall circuit typically has resistance in the range of 30Ω to 80Ω. Thus, impedance measurements are correspondingly insensitive to significant changes in cable or coil impedance.
The difficulty in detecting a partial failure with present electrical testing may also be appreciated from the following example. Consider a fracture in the conductor leading to the RV coil (RV conductor). The typical SVC coil has an impedance on the order of 60Ω through to another electrode or the CAN, while the cable connection impedance is only about 1.5Ω (from the combination of two cables in parallel, each having impedance of 3Ω). Thus, the complete fracture of one of the two cables would result in an impedance increase of only 1.5Ω, which is far lower than the typical (5Ω to 10Ω) serial impedance changes seen chronically.
In addition to limited sensitivity, present methods for diagnosing lead conductor anomalies have limited specificity resulting in false positive diagnostics. Evaluation of false positive diagnostics adds cost and work to medical care and may contribute to patient anxiety. If a false-positive diagnostic is not diagnosed correctly, patients may be subject to unnecessary surgical lead replacement with its corresponding risks.
Any clinical method for detecting conductor anomalies in implanted leads must make measurements while the conductor and lead are in the body. Typically, the measuring circuit includes the conductor-tissue interface in the body. Thus, the measured values will depend both on the behavior of the conductor being evaluated and the conductor-tissue interface.
Existing technology for diagnosis of conductor anomalies in an implantable medical device is believed to have significant limitations and shortcomings. What is desired are method and apparatus that could analyze and identify implantable cardiac lead conductor anomalies at the subclinical stage, before they present as a clinical problem, and do so with a high sensitivity and specificity that minimizes false positives for implantable cardiac lead conductor anomalies.